This invention concerns in general improved method and apparatus for waste water treatment, and in particular concerns the treatment of waste water containing heavy metals.
Industrial waste waters commonly include a variety of contaminants which require treatment (i.e. removal) even before the waste water can be discharged from the plant site. The nature of the waste water contaminants is in large part dependent on the commercial processes practiced in the plant. Accordingly, there is great variety in the nature of waste water contaminant problems. Moreover, the matrix (i.e., makeup) of waste water even at a given commercial site will usually vary, sometimes dramatically, with changes in production or the like.
Particular industries, for example such as those relating to metal plating, metal finishing, or circuit board manufacturing activities, generate waste water with heavy metals (e.g., copper, nickel, etc.) and other metals in solution with such waste water. The commercial activities themselves may inherently generate heavy metals which are chelated and/or complexed for purposes of the commercial activity (e.g., metal plating) itself. Chelating and/or complexing tends to cause such metals to remain in solution, and thus require special attention for their removal.
During the typical course of plant activity, heavy metal concentration in the waste water is highly variable. While concentration variations can in general be expected, monitoring of and reacting to specific variations is problematic. Concentrations of heavy metals may typically vary from a few parts per million to several hundred parts per million, even in a very short time, such as a matter of minutes.
Not only do concentration levels vary drastically, but extreme variations can be experienced with respect to the matrix (both in identity and nature, e.g., chelated versus non-chelated) of heavy metals present.
In general, it is known to add (i.e., feed) various precipitating agents to waste water to precipitate such heavy metals for their removal from the water. The amount of such precipitating agents required (i.e. consumed) in the course of precipitating such heavy metals of course depends on the degree of presence of such heavy metals in solution with the waste water. Since effective real time monitoring of heavy metal concentration levels has heretofore generally proven difficult, such treatment chemical feeding (i.e. the feed rate of precipitating agents) is typically set at a compromise level, such as for precipitating the maximum expected concentration of heavy metals. Such a compromise setting creates an excess amount of sludge, which sludge may often be classified as a hazardous waste. Moreover, since the cost of the treatment chemicals is not insignificant, wasteful overfeeding thereof is costly.
Operators have been known to attempt periodic checks to manually detect the level of metals entering the waste water (i.e., assess the expected concentrations), and adjust the chemical feed rate accordingly. However, such a manual adjustment merely alters the set feed rate in accordance with periodic reassessments of the anticipated maximum concentration, and does nothing to eliminate excess sludge production and excessive and costly chemical usage caused by differences between actual concentration levels and the anticipated maximums thereof. Moreover, short term spikes can still occur, meaning that inadequately treated waste water can be nonetheless discharged. Such occurrences are particularly problematic where applicable laws regulate the permissable discharge concentration levels, such as to certain fractional parts per million or certain parts per million.
In some industrial settings, anticipation of heavy metal concentrations in the waste water may be relatively less "predictable". For example, a totally unexpected occurrence of heavy metals in the waste water can go unchecked, thereby causing the plant to exceed permissible discharge levels. For example, maintenance personnel might empty mop buckets or the like containing chelated heavy metals picked up from the floor of the facility, which could cause a heavy metal concentration spike in the waste water at a time whenever commercial activity in the plant is nil, and precipitating agent feed pumps may be switched off. The plant is nonetheless responsible for its waste water discharge, though no effective continuous monitoring systems for preventing such undesirable discharges may be available.
It is generally known that certain metals in solution in waste water may be precipitated therefrom by controlling the pH level of the waste water. For example, non-chelated and non-complexed metals in particular may be in various degrees precipitated in such manner. Automatic controllers are generally available which function to probe the waste water for its pH level, and automatically pump treatment chemicals accordingly to the waste water so as to adjust its pH level within an established deviation from a pre-selected setpoint. One example of such a controller is the Model 5 proportional pH pump controller, made by Chem-Tech International, Inc., of 92 Bolt Street, Lowell, Mass., 01853. While such a controller may be effective for metals which may be precipitated through such pH inducement, heavy metals which are chelated and/or complexed generally will not be precipitated with such pH level control. Thus, the monitoring and treatment problems noted above persist, and may be compounded where a changing mix of chelated and non-chelated metals is presented for treatment.
Another aspect of waste water treatment problems where both such types of metals are in solution (i.e., which can and can not be practically precipitated through pH inducement) is that use of a precipitating agent can precipitate both such types of metals. However, unnecessary sludge production is caused by precipitating metals in such a manner which could have otherwise been precipitated through pH level control (as generally discussed above). Again, the amount of precipitating agent consumption is also a factor.
In addition to the availability of known pH level control generally outlined above, at least one other generally known method, involving a so-called oxidation reduction potential probe, attempts to address precipitating agent usage. Such a probe is typically used to detect the presence of excess (i.e. un-consumed) precipitating agent at a phase of a waste water treatment program after all the metal is removed. One particular limitation of such a system is that it cannot distinguish between, for example, chelated and non-chelated metals, and must therefore feed precipitating agent until there is an excess of such agent present in the water. Feed control feedback also is derived from detected excess agent, not from information relative remaining metal in solution to be precipitated. Thus, there is no effective prevention of excess sludge generation or wasteful chemical usage.
Another limitation of a waste treatment system utilizing an oxidation reduction potential (ORP) probe is that the probe operation involves an electrical measurement which is affected by changes in the pH level of the waste water, the amount of total dissolved solids therein, and the amount of chelated metal in the waste water. Thus, an ORP probe system is inherently ineffective for use in providing close control of the feeding of chemical treatment solutions into waste water treatment systems.